Electrical Engineering Project Report
Assembly of Audio Cassette Player Kit Model AK-200/TR-18K
Elenco Electronics, Inc.
Part I: PC Board Assembly Notes
The assigned task was to build the Cassette Player’s PC board, attach it to the tape deck electrically by wiring, and insert batteries to operate the entire unit. The building of the PC board proved to be a delicate task involving at least intermediate level skills in soldering.
On the first laboratory day available for assembly, both partners met to discuss the nature of the task, examine the kit, check the components against a master list supplied by Elenco and begin assembly.
It was immediately determined that the tip of the soldering iron in the EE building was too large and far too broad to use on the micro-wiring and holes of the miniature circuit board. This oversized lab equipment caused several connectors to be unintentionally soldered together. When both partners attempted to remove faulty connections with the solder vacuum-sucker, this approach did not resolve the problem.
In fact, the broad tip of the vacuum device was unable to enter the confined space when repeated attempts were made to remove misplaced solder at various connection junctions. Basically all attempts resulted in complicating the situation. Even though problems were encountered the partners still managed to keep soldering and complete 50% of the project.
On the second laboratory day, the project was moved to one of the partner’s garage where more miniaturized soldering equipment was available. Further grinding of the soldering iron tip improved the ability of the tool used to place solder as desired. Resin was used to facilitate soldering when required. After two hours suitable progress was made and the circuit board portion of the project was completed with only minor complications.
On the third laboratory day, both Lab Partners met and placed the finishing touches on the project. A few problems with the previous soldering prevented the proper operation of the cassette machine, all major components of which had now been assembled completely. The entire unit had to be taken back to the garage and both partners rechecked everything from Step 1, in order to see what was wrong with the soldering and circuit board. No major errors appeared to have been made, but the audio still did not function.
On the fourth laboratory day, a new circuit board was obtained and completely resoldered. The audio still did not function properly, although all electro-mechanical functions did operate properly (tape drive, feed and take-up reels, start-stop functions, etc). This may be due to defective headphones or to an improper connection on the circuit board, undetected by the partners who, nonetheless, had rechecked everything twice after the re-assembly process. The malfunctioning unit will be placed at the disposal of the Professor for examination and comment.
Part II: Theoretical Analysis of Operation
There are two primary areas pertaining to the Cassette Player that have to be examined in terms of their theoretical and operational bases. The first is in the area of electro-mechanical theory and the second is in the sector of the device’s audio function, namely the theory of ferromagnetism.
The main problem associated with the electro-mechanical aspect of the cassette player involves keeping the motor voltage constant as the battery voltage continues to drop. There is a motion control section of the player containing a "voltage divider unit" and a motor control IC (reference voltage and op-amp). These devices keep the voltage constant even as battery power drops, perhaps incrementally by .5V depending on use of player. By maintaining a constant 1.3 V current across the divider, the motor voltage (and hence speed) will remain constant. For proper playing of the tape, of course, motor speed must not vary.
As might be expected, the audio portion of the cassette player operates on an entirely different theory. A magnetic field can be used to generate voltage. A magnet can be inserted into a coil of wire and will produce current. The voltage of this current is in proportion to the magnetic flux, in other words to the rate of change of the ‘lines’ linking the coil together. If the magnet is inserted into the coil at a fast rate, more voltage is produced, since more flux is involved. But when no rate of change occurs the voltage is zero. During withdrawal of the magnet from the coil reverse polarity voltage is created, again proportional to the speed of withdrawal.
This same principle, with important variations, is used in the recording of sound on iron oxide or chromium dioxide coated plastic tape. When this specially coated tape is subjected to variations in an external magnetic field, individual electrons align themselves in a distinctively non-random manner. Non-coated tape would result, of course, in random alignment. Hard magnetic material produces better results in aligning electrons on treated tape. By varying the strength of the magnetic field created, electrons form certain patterns on the tape. The tape head places a fluctuating magnetic field in proximity to the tape.
Magnetic flux, in the North to South direction, magnetizes the iron-oxide (or chromium dioxide) surface of the tape leaving permanent, but erasable, patterns converted to sound via the playback head. This ferromagnetic, hysteresis loop technology was developed in the 1930s and 40s first in the form of wire recorders. It was later converted to reel-to-reel tape recorders, and was still later reduced in size to smaller cassette decks and players. The quality of sound reproduced has been greatly improved over the decades through the use of AC Bias and Equalization technology, among other innovations.